82C54 Datasheet, PDF (6/17 Pages) Intel Corporation – CHMOS PROGRAMMABLE INTERVAL TIMER

82C54
Possible Programming Sequence (Continued)
LSB of Count - Counter 1
LSB of Count - Counter 0
MSB of Count - Counter 0
MSB of Count - Counter 1
MSB of Count - Counter 2
A1
A0
0
1
0
0
0
0
0
1
1
0
Possible Programming Sequence
Control Word - Counter 2
Control Word - Counter 1
Control Word - Counter 0
LSB of Count - Counter 2
MSB of Count - Counter 2
LSB of Count - Counter 1
MSB of Count - Counter 1
LSB of Count - Counter 0
MSB of Count - Counter 0
A1
A0
1
1
1
1
1
1
1
0
1
0
0
1
0
1
0
0
0
0
Possible Programming Sequence
A1
A0
Control Word - Counter 1
1
1
Control Word - Counter 0
1
1
LSB of Count - Counter 1
0
1
Control Word - Counter 2
1
1
LSB of Count - Counter 0
0
0
MSB of Count - Counter 1
0
1
LSB of Count - Counter 2
1
0
MSB of Count - Counter 0
0
0
MSB of Count - Counter 2
1
0
NOTE: In all four examples, all counters are programmed to
Read/Write two-byte counts. These are only four of many
programming sequences.
A new initial count may be written to a Counter at any time
without affecting the Counter’s programmed Mode in any way.
Counting will be affected as described in the Mode definitions.
The new count must follow the programmed count format.
If a Counter is programmed to read/write two-byte counts,
the following precaution applies. A program must not transfer
control between writing the first and second byte to another
routine which also writes into that same Counter. Otherwise,
the Counter will be loaded with an incorrect count.
Read Operations
It is often desirable to read the value of a Counter without
disturbing the count in progress. This is easily done in the
82C54.
There are three possible methods for reading the Counters.
The first is through the Read-Back command, which is
explained later. The second is a simple read operation of the
Counter, which is selected with the A1, A0 inputs. The only
requirement is that the CLK input of the selected Counter
must be inhibited by using either the GATE input or external
logic. Otherwise, the count may be in process of changing
when it is read, giving an undefined result.
Counter Latch Command
The other method for reading the Counters involves a spe-
cial software command called the “Counter Latch Com-
mand”. Like a Control Word, this command is written to the
Control Word Register, which is selected when A1, A0 = 11.
Also, like a Control Word, the SC0, SC1 bits select one of
the three Counters, but two other bits, D5 and D4, distin-
guish this command from a Control Word.
.
A1, A0 = 11; CS = 0; RD = 1; WR = 0
D7 D6 D5 D4 D3 D2 D1 D0
SC1 SC0 0
0
X
X
X
X
SC1, SC0 - specify counter to be latched
SC1 SC0
COUNTER
0
0
0
0
1
1
1
0
2
1
1
Read-Back Command
D5, D4 - 00 designates Counter Latch Command, X - Don’t Care.
NOTE: Don’t Care bits (X) should be 0 to insure compatibility with
future products.
The selected Counter’s output latch (OL) latches the count
when the Counter Latch Command is received. This count is
held in the latch until it is read by the CPU (or until the Counter
is reprogrammed). The count is then unlatched automatically
and the OL returns to “following” the counting element (CE).
This allows reading the contents of the Counters “on the fly”
without affecting counting in progress. Multiple Counter Latch
Commands may be used to latch more than one Counter.
Each latched Counter’s OL holds its count until read. Counter
Latch Commands do not affect the programmed Mode of the
Counter in any way.
If a Counter is latched and then, some time later, latched
again before the count is read, the second Counter Latch
Command is ignored. The count read will be the count at the
time the first Counter Latch Command was issued.
With either method, the count must be read according to the
programmed format; specifically, if the Counter is pro-
grammed for two byte counts, two bytes must be read. The
two bytes do not have to be read one right after the other;
read or write or programming operations of other Counters
may be inserted between them.
Another feature of the 82C54 is that reads and writes of the
same Counter may be interleaved; for example, if the
Counter is programmed for two byte counts, the following
sequence is valid.
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